12 LIGHT IN RELATION TO TREE GROWTH. 
adapt themselves to their needs; therefore, open-grown trees usually 
appear the more thrifty. 
The primary cause of this difference in tolerance must be sought in 
the anatomical structure and inherent qualities of the leaves and 
chlorophyll. Since, however, the anatomical structure of the leaves 
and even the character of the chlorophyll may be influenced by en- 
vironment, the tolerance of trees is not a fixed quality, but is subject 
to variation. Each species, however, inherently requires a certain 
amount of light, which can not be changed by any environment. 
LUBIMENKO’S EXPERIMENTS. 
The latest experiments by Prof. Lubimenko?! established with 
sufficient accuracy the difference in the sensitiveness of the chloro- 
plasts of different tree species. The species with which he experi- 
mented were Scotch pine (Pinus sylvestris), noble fir (A bées nobilis) , 
white birch (Betula alba), and linden (Tilia cordata). Of.these, 
pine and birch are light-needing species and stand close to each 
other in this respect. The fir and basswood are classed as tole- 
rant, and are also fairly similar in their demands upon light. More- 
over, the anatomical structures of the leaves of pine and fir are very 
similar, and of birch and basswood practically identical. 
These experiments brought out clearly the following points: 
(1) The initial hght intensity at which assimilation begins varies 
with the species, since the fir and basswood began assimilation at 1/5 
of the hght intensity at which pine and birch began to assimilate. 
(2) If it be accepted that the light intensity at which assimilation 
equals respiration is the minimum for the existence of the leaf, then 
birch appears to be the most light-demanding species, followed by 
pine, basswood, and finally fir. 
(3) With increase in light, the assimilation at first increases in all 
four species, but in direct sunlight, when the rays strike the surface 
of the leaves perpendicularly, the pine and birch still continue to 
show an increase in assimilation, while the fir and basswood show a 
decrease. This may be seen in Table 2. 
TABLE 2.—Relative amounts of carbon dioxide absorbed by 1 gram of leaf during 
one hour of work. 
Pine. Fir. Birch. Basswood. 
Direct rays of the sun striking the surface of the leaf at 
ancacute-angle cocoa Ge sete i tee blo opera tester 49. 74 53. 10 68. 08 68. 71 
Direct rays of the sun striking the surface of the leaf 
perpendicularly. <.132~ see ceine esses See me eee eee eee 56. 83 38. 50 75. 69 43. 21 
Since the anatomical structures of the leaves of pine and fir are 
very similar, and of birch and basswood almost identical, this differ- 
ence in assimilation must be explained exclusively by the difference 
1 Carried on in 1904 at the Laboratoire de Biologie Vegetale, in Fontainebleau, France. 
